1. Field of the Invention
The present invention relates in general to a power transmission shaft unit for use in wheeled motor vehicles, and more particularly to a power transmission shaft unit such as propeller shaft unit, drive shaft unit or the like of the vehicle.
2. Description of the Prior Art
Some of the power transmission shaft units are of a type that comprises a universal coupling and a hollow connecting member which are connected in line. These two members have mating surfaces at their axial connecting ends. One of the mating surfaces is formed with an annular ridge that is coupled with an annular recess formed in the other of the mating surfaces, and the universal coupling and the hollow connecting member are joined with the aid of connecting bolts.
However, sometimes, the above-mentioned joining tends to cause the two joined members to constitute therebetween an isolated air space. Thus, under operation of the transmission shaft unit, heat inevitably produced in the universal coupling transmits to the isolated air space to heat and thus expand the air therein. However, the expansion of air tends to apply a marked stress to sealing members, such as a dust boot and the like, causing the sealing members to have an undesired permanent set. One of measures for solving this undesirable matter is to provide the isolated air space with an air discharge passage. This measure is described in Service Manual of MITSUBISHI PAJERO issued from MITSUBISHI MOTOR CO., in July, 1993.
However, even this measure has failed to provide a satisfied performance due to its inherent construction. That is, in this known measure, foreign matters, such as muddy water and the like tend to enter the enclosed space through the air discharge passage, which has an undesired influence on lubrication of friction parts in the shaft unit.